What causes white smoke when vehicle is idling in cold weather?

Answer 1

Answer 1The white smoke is basically the same as you breating out in cold weather. It is warm air hitting cold air that makes it visible. The "smoke" is is CONDENSED WATER VAPOR, which is a byproduct of the fuel combustion process. The key is the presence of WATER VAPOR MOLECULES [too small to see, and therefore invisible]in the vehicle exhaust gasses that upon contact with the cold air, condense into water DROPLETS which ARE VISIBLE. Another well known example of this phenomenon is airplane "contrails," which occur at very high altitudes, and very cold temperatures. Most young folks think this is due to "jet" planes, but any piston engine aircraft will produce contrails at the right ambient temperatures. To see this, pay attention to high altitude bomber scenes in WWII movies, like "12 O'clock High" as one example.